A conventional subsea wellhead assembly includes a high pressure wellhead housing which supports one or more casing hangers located at upper ends of strings of casing extending into the well. A tubing hanger lands in the wellhead housing above the casing hanger and supports a string of production tubing that extends through the smallest diameter casing. The tubing hanger has a production bore which is offset slightly from the longitudinal axis. An annulus bore also extends through the tubing hanger, parallel to and offset from the axis, for communicating the tubing annulus to above the tubing hanger. The annulus bore is needed during installation of the tubing hanger and tubing to establish circulation down the tubing and back up the annulus. After the well has been completed, a removable plug is installed in the annulus bore, then a production tree is mounted to the wellhead housing. Access through the production tree to the tubing may be made for various workover operations that are needed. Various production valves and chokes will be mounted to the tree.
In the last few years, operators have begun installing a different type of wellhead assembly, referred to generally as a horizontal tree. In a horizontal tree, the tubing hanger lands in the tree, not in the wellhead housing located below the tree. The tubing hanger has a lateral flow passage extending from its vertical flow passage. The lateral flow passage registers with a lateral flow passage extending through a sidewall of the tree. With the horizontal tree, a tubing hanger can be pulled through the horizontal tree without removing the tree. Various production valves and chokes are mounted to the horizontal tree in the same manner as with a conventional tree.
In both types of trees, the valves and chokes are not readily accessible from the surface. Some work can be performed with a remote operated vehicle. If the entire valve needs to be replaced, it may be necessary to retrieve the tree. This is a time consuming and expensive task, particularly with a horizontal tree because the tubing must be pulled first.